Electrochemical batteries classically include pairs of oppositely charged plates (positive and negative), and an intervening electrolyte to convey ions from one plate to the other when the circuit through the battery is completed. This is a very well developed and active art, but after decades of steady effort and improvement, batteries still remain a principal impediment to the employment of electricity as a motive force in many practical applications.
A battery's capacity to deliver electrical current is a straight-line function of the surface area of its plates which is contacted by the electrolyte. A flat plate constitutes a lower limit, which is frequently improved by sculpting its surface. Waffle shapes are well-known, for example. There is a physical limitation to what can be done to "open-up" the surface of the plates, because these plates must resist substantial mechanical stringencies such as vibration and acceleration, and must be strongly supported at their edges. Thus, plates which are rendered delicate by casting or molding them into shapes which have thin sections are not a viable solution to increase the surface area of the plates. Also, such plates are subject to erosion and loss of material, thereby further reducing the strength of the plate over the life of the battery. A tempting solution is to use a woven screen for a plate. However, screens can be bent, usually on two axes. Especially after significant erosion they do not have sufficient structural strength. A battery is destroyed if a screen or plate collapses or contacts a neighbor.
Despite the inherent potential structural disadvantages, it is a valid objective to attempt to increase the area exposed to the electrolyte by giving access to interior regions of a plate. Otherwise the entire interior of the plate serves as no more than an electrical conductor and support for the surface of the plate. Holes through the plate can in fact increase surface area by the difference between their area removed from the surface and the added area of their walls. There is an obvious limitation to this approach.
A benefit in addition to increased surface area which could be obtained with an open-structured plate is the storage of electrolyte within the envelope of the plate. In turn, for a given amount of electrolyte volume, the gross volume of the battery can be reduced by the amount which is stored in the plates, rather than in the spacing between plates. Evidently the problem is one of increasing the surface area of the plates without compromising their strength.
This invention provides a rigid plate structure with substantial open passages formed by an assembly of rigid elongated links rigidly joined together at intersections to form a continuous monolithic body braced in all directions to resist compression, elongation, bending, vibration and acceleration. The links provide this structural rigidity, and also act as boundaries of (or impediments in) the passages, and form a substantial area exposed to the electrolyte. Such a structure is frequently called "reticulated". When this structure is made of a metal that takes part in the battery reactions, it forms the plate itself. When it is covered by a compound that is involved in the reactions, often applied as a paste, the structure comprises a substrate support, and is thereby only a part of the plate. It establishes the shape of the surface.
The use of the resulting structurally rigid plates has the further advantage that they can be placed closer together because of their rigidity, thereby reducing the size of the battery, and the quantity of electrolyte which is required. At once this increases the power density for both weight and volume of the battery. In addition, a porous non-conductive spacer, preferably with similar geometry, can be placed in abutment with the plates to provide a solid reinforced structure.
Stagnation is another problem faced by all batteries. Because the mechanism of ion transfer requires migration of ions relative to the exposed surfaces, when the electrolyte at its interface with the plate is depleted, replacement by ion migration is slow. In this invention, interface conditions can be improved by ultrasonic vibration of the plates, or of the electrolyte, or of both, which causes physical movement at the interface. This low energy vibration is not effective in solid plates, and is intolerable in very weak plates. However with the structure of the plates according to this invention, ultrasonic vibration of the plates or of the electrolyte (or of both) at the interface is practical and effective.
The use of ultrasonic vibration enables the use of a very convenient gelled electrolyte. Some such gels are poorly conductive when jelled, but when subjected to such energy they liquify and become more conductive. Their jelled condition makes them more stable when handled. By utilizing a plate which can be responsive to such vibration without damage, then the advantages of such an electrolyte become available, a particularly useful feature as to the electrolyte which is housed with the passages in the plates.